The growth cone behaviors that are involved in the generation of neuronal cytoarchitecture are apparently regulated in quite specific ways by Ca e+. Neurotransmitters and electrical activity, well knoum for their roles in information coding, have recently been shown to affect growth cone motility by mechanisms linked to Ca e+. Ca e+ may therefore act as a common integrator of environmental cues that influence neurite outgrowth and synaptogenesis, and in this way may play a key role in the establishment and modulation of brain circuitry.

Defining the essential features that comprise the neuronal growth cone is no simple task. The mental image of the'living battering ram'suggested by Cajal's 1 first description of the growth cone must certainly have provided strong motivation for the pioneering work of Harrison 2 who first watched living growth cones with his newly developed method of tissue culture. In fact, much of our contemporary view of the growth cone is based upon its form in tissue culture. One sees at the terminal of extending neurites a broad flattened lameUipodium often tipped by numerous spike-like filopodia. In situ, however, a variety of growth cone shapes can be observed 4'5. Even in the relatively simple and highly defined system of the developing Daphnia eye, only one of the several axons coursing together displays a'classical'growth cone form, whereas the others have no prominent lamellipodia or filopodia 6. Despite this perplexing variety of forms, certain features must be common to all elongating neurite tips: they must add membrane and they must construct and stabilize cytoskeletal scaffolding. It seems most useful to define all elongating neurite tips as growth cones and then to investigate the different behaviors exhibited by cones in which related structures have differing degrees of prominence. For example, lamellipodia may form in reaction to different surface adhesiveness, and filopodia may act as sensory transducers that convey information about distant environmental cues 7. Eventually we will need to understand the functional roles of each of the different growth cone components.